N-Arylethyl-maleimides that can be coupled with S35 and H3-labeled sulfanilic acid to form radioactively labeled sulfhydryl reagents have been synthesized and will be used to study the effects of agonist-dependent conformational changes on the reactivity of protein SH groups of adrenergic receptors in pineal bodies and smooth muscle. In an effort to provide material sufficiently enriched in receptor content to make such studies practical, beta-adrenergic receptors of pineal bodies have been fractionated by differential centrifugation and salt extraction.